Searching for a solution to this really annoying problem in Google have given me this page.
In case somebody else is having this problem too, I just want to share this easy fix that actualy worked for me:
Try unchecking MP3 options in QT MIME settings. Then uncheck the Mp3 setting in WMP (Windows Media Player). Hit apply, close WMP. Reopen and then check Mp3 file type and apply. Close WMP. Try to open an mp3 files in your browser. Give it a try. You might have to reboot.
This solution was found on the Apple forum: http://discussions.apple.com/thread....79734􇦶
I did'nt needed a reboot to get this done.
If you want Winamp to play these mp3 files, after having done it with WMP, do it with Winamp.